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PHP Accessing Parent Class Variable

Posted by: admin November 30, 2017 Leave a comment

Questions:
class A {
    private $aa;
    protected $bb = 'parent bb';

    function __construct($arg) {
       //do something..
    }

    private function parentmethod($arg2) {
       //do something..
    }
}

class B extends A {
    function __construct($arg) {
        parent::__construct($arg);
    }
    function childfunction() {
        echo parent::$bb; //Fatal error: Undefined class constant 'bb' 
    }
}

$test = new B($some);
$test->childfunction();

Question:
How do I display parent variable in child?
expected result will echo ‘parent bb’

Answers:
echo $this->bb;

The variable is inherited and is not private, so it is a part of the current object.


Here is additional information in response to your request for more information about using parent:::

Use parent:: when you want add extra functionality to a method from the parent class. For example, imagine an Airplane class:

class Airplane {
    private $pilot;

    public function __construct( $pilot ) {
        $this->pilot = $pilot;
    }
}

Now suppose we want to create a new type of Airplane that also has a navigator. You can extend the __construct() method to add the new functionality, but still make use of the functionality offered by the parent:

class Bomber extends Airplane {
    private $navigator;

    public function __construct( $pilot, $navigator ) {
        $this->navigator = $navigator;

        parent::__construct( $pilot ); // Assigns $pilot to $this->pilot
    }
}

In this way, you can follow the DRY principle of development but still provide all of the functionality you desire.

Questions:
Answers:
class A {
    private $aa;
    protected $bb = 'parent bb';

    function __construct($arg) {
       //do something..
    }

    private function parentmethod($arg2) {
       //do something..
    }
}

class B extends A {
    function __construct($arg) {
        parent::__construct($arg);
    }
    function childfunction() {
        echo parent::$this->bb; //works by M
    }
}

$test = new B($some);
$test->childfunction();`

Questions:
Answers:

With parent::$bb; you try to retrieve the static constant defined with the value of $bb.

Instead, do:

echo $this->bb;

Note: you don’t need to call parent::_construct if B is the only class that calls it. Simply don’t declare __construct in B class.

Questions:
Answers:

Just echo it since it’s inherited

echo $this->bb;

Questions:
Answers:

$bb has now become the private member of class B after extending class A where it was protected.

So you access $bb like it’s an attribute of class B.

class A {
    private $aa;
    protected $bb = 'parent bb';

    function __construct($arg) {
       //do something..
    }

    private function parentmethod($arg2) {
       //do something..
    }
}

class B extends A {
    function __construct($arg) {
        parent::__construct($arg);
    }
    function childfunction() {
        echo $this->bb; 
    }
}

$test = new B($some);
$test->childfunction();

Questions:
Answers:

all the properties and methods of the parent class is inherited in the child class so theoretically you can access them in the child class but beware using the protected keyword in your class because it throws a fatal error when used in the child class.
as mentioned in php.net

The visibility of a property or method
can be defined by prefixing the
declaration with the keywords public,
protected or private. Class members
declared public can be accessed
everywhere. Members declared protected
can be accessed only within the class
itself and by inherited and parent
classes. Members declared as private
may only be accessed by the class that
defines the member.

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